Research Objective is an English translation of Galen's most important clinical treatise ON THE DISEASED PARTS OF THE HUMAN (DE LOCIS AFFECTIS) from the Greek text of the Kuehn edition of Galen's works. This treatise contains an analysis of general diagnostic principles, a description of many internal, neurological and psychiatric diseases, arranged according to organ systems. Unlike other clinical treatises of antiquity Galen's book stresses the relation of clinical observations to his knowledge of structure and function of the affected organs and his experimental studies on these problems. It has never been translated into English nor has it been annotated, or analysed in depth. Explanatory footnotes will be provided on each page and a general introduction on Galen's concepts. It will supplement as source material my previously published books which gave an analysis of "Galen's system of physiology and medicine."